Rigid polyurethane and polyisocyanurate foams are produced using cell-stabilizing additives to ensure a fine-celled, uniform and low-defect foam structure and hence to exert an essentially positive influence on the performance characteristics, particularly the thermal insulation performance, of the rigid foam. Surfactants based on polyether-modified siloxanes are particularly effective and therefore represent the preferred type of foam stabilizer. Various publications already describe such foam stabilizers for rigid foam applications.
EP 0 570 174 B1 describes a polyether siloxane of the structure (CH3)3SiO[SiO(CH3)2]x[SiO(CH3)R]ySi(CH3)3, the R substituent of which consist of a polyethylene oxide linked to the siloxane through an SiC bond and end-capped at the other end of the chain by a C1-C6 acyl group. This foam stabilizer is suitable for producing rigid polyurethane foams using organic blowing agents, particularly chlorofluorocarbons such as CFC-11.
The next generation of chlorofluorocarbon blowing agents are hydrochlorofluorocarbons such as HCFC-123 for example. When these blowing agents are used for rigid polyurethane foam production, it is polyether siloxanes of the structural type (CH3)3SiO[SiO(CH3)2]x[SiO(CH3)R]ySi(CH3)3 which are suitable according to EP 0 533 202 A1. The R substituent in this case consist of SiC-bonded polyalkylene oxides which are assembled from propylene oxide and ethylene oxide and can have a hydroxyl, methoxy or acyloxy function at the end of the chain. The minimum proportion of ethylene oxide in the polyether is 25 percent by mass.
EP 0 877 045 B1 describes analogous structures for this production process which differ from the first-named foam stabilizers in that they have a comparatively higher molecular weight and have a combination of two polyether substituents on the siloxane chain.
A more recent development in the production of rigid polyurethane foams is to dispense with halogenated hydrocarbons as blowing agents entirely and to use hydrocarbons such as pentane instead. EP 1 544 235 thus describes the production of rigid polyurethane foams using hydrocarbon blowing agents and polyether siloxanes of the already known structure (CH3)3SiO[SiO(CH3)2]x[SiO(CH3)R]ySi(CH3)3 having a minimum chain length for the siloxane of 60 monomer units and different polyether substituents R, the blend average molecular weight of which is in the range from 450 to 1000 g/mol and the ethylene oxide fraction of which is in the range from 70 to 100 mol %.
As mentioned, the use of foam stabilizers serves to improve the performance characteristics of polyurethane foams, for example their insulation performance and their surface characteristics. In addition, however, improved processing characteristics would also be desirable. Apparatus used for applying the reaction mixture by low or high pressure foaming machines may clog up at the discharge nozzle because of a build-up of reaction mixture in the process of reacting, particularly in continuous applications and when used as spray foam. This is particularly likely with highly catalyzed/reactive systems because of their high rate of reaction. This may lead to reduced output or complete clogging of the discharge orifice. This in turn entails idle times and short cleaning and maintenance intervals.
The problem addressed by the present invention was therefore that of providing foam stabilizers enabling the production and processing of polyurethane and/or polyisocyanurate foams without retaining one or more of the disadvantages known from the prior art, e.g. the clogging of discharge orifices in processing apparatus, while preferably not exerting any negative influence on foam properties.
The problem was found to be solved, surprisingly, by compositions comprising two different varieties of polyether siloxanes as described hereinbelow and in the claims.